Active member

Most users who register multiple accounts do so because they have forgotten about their old account or don't know how to operate the site.
If you're worried about users registering new accounts for malicious purposes you should consider enabling admin approval of accounts. You can then check each new account for the usual signs such as a temporary email, proxy IP, or an IP matching other users recently.

Matching IPs from everyone would look something like...
A bunch of accounts from ISPs that have highly dynamic or shared IPs.
And mobile IPs which are less plentiful and also highly dynamic.
And shared networks (ex. schools) having many users on a single IP.
And things like Opera Mini and open proxies would also cause a lot of accounts to be linked together.
And many IPs from less dynamic pools matching months to years apart.

I don't think you'd get a good signal to noise ratio by matching all users' IPs together.